1. Field of the Invention
The present invention relates to an electron emitting source such as a thermionic cathode using lanthanum hexaboride or cerium hexaboride monocrystals used in electron microscopes, critical dimension scanning electron microscopes and electron beam exposure apparatus, and a method of manufacturing such an electron emitting source.
2. Description of the Related Art
An example of an electron emitting source used in equipment applying electron beams such as electron beam lithography apparatus and electron microscopes is one using an element functioning as an electrically heatable heater of highly crystalline carbon or the like, an electron emitting source tip (hereinafter referred to simply as a “tip”) composed of a rare earth hexaboride that radiates electrons sandwiched by the aforementioned heater, and an electrically conductive support holding the aforementioned tip together with the heater (see Patent Document 1).
More specifically, as shown for example in FIG. 1, terminals (2a and 2b) and electrically conductive supports 3 are attached to an insulator 1, at the ends thereof, the supports 3 sandwich a tip 4 for emitting electrons and heaters 5 in the form of rectangular solids positioned at both side surfaces of the tip 4, and thereby hold the tip 4 and heaters 5. LaB6 or the like can be used as the material of the electron emitting source tip 4.
The heater may be an element of a carbonaceous material such as so-called hot press carbon (see Patent Document 1) which is obtained by hot-pressing a furan resin or the like, or a thermolytic carbon (see Patent Document 2). The above elements have anisotropic volume resistivity, and are usually shaped in the form of a rectangular solid, which is sandwiched by the electrically conductive supports 3 such as to allow passage of electricity in the direction of high volume resistivity.    Patent Document 1: JP-B S60-23456    Patent Document 2: JP S47-25911
With the electron emitting source shown in FIG. 1, by passing a DC current through the terminals (2a and 2b) in a high vacuum, the tip 4 can be heated to a predetermined temperature by the heaters 5, and by further placing the tip 4 in an electric field, electrons can be extracted from the tip 4, enabling it to be used as an electron emitting source in electron beam lithography apparatus or electron microscopes.
Normally, when using LaB6 or the like as the electron emitting source tip 4, it is heated to 1400-1500° C.